This invention relates to laser Doppler velocity sensors of the type used to measure the velocity of a number of aerosol or other particles in an atmospheric volume and, more particularly, to such a sensor which is capable of effectively measuring the three velocity components of said particles from a remote site.
Laser Doppler sensors are finding increasing use to optically measure such things as the velocity of flow of air or liquids. Such a velocimeter can provide a dynamic measurement of flow velocity without requiring a probe or other structure to be located in the flow and disturbing it. For example, a so-called laser velocimeter provides a velocity measurement of a fluid by focusing one or more coherent beams of laser light onto a point within a fluid stream containing particles having diameters on the order of the laser light wavelength. Such particles will "scatter" the light radiation by the Mie scattering process, and shift its wavelength by an amount dependent upon its velocity and, hence, the velocity of the fluid. Such Doppler shifting of the wavelength is measured by various techniques to provide a scaler measurement of the velocity of the particle along at least one path.
Many modern laser Doppler velocimeters are of the so-called "dual scatter" kind. Such instruments include a pair of coherent beams which are focused to a crossing point at the location at which it is desired that the velocity of the particle be measured. A fringe field is thereby formed at the beam crossing location due to beam interference. As a particle moves through such a fringe field, the light intensity scattered therefrom is modulated at a frequency which is proportional to the scaler component of the velocity of such particle in a direction which lies in a plane normal to the bisector of the beam angle at their point of incidence on the particle and also in the plane of the beams. Collection of such intensity-modulated scattered light radiation by a photodetector provides a measurement of such velocity component.
In general, dual-scatter velocimeters measure only the component of the velocity of the particle perpendicular to the beam angle bisector and in the plane of the beams. Thus, to obtain a true indication of the flow velocity, a plurality of velocimeter systems must be provided, all focused on the same location in the flow to separately measure different velocity components which can then be combined to provide the actual velocity. This utilization of a plurality of systems not only increases the expense due to system duplication, but also reduces the accuracy of the velocity determination due to the adding of potential errors in the systems. Moreover, in some of such systems, each of the detectors receives scatter from more than one of the incident beams, with the result that expensive, special discrimination electronics or optics must be used before reliable measurements can be obtained. In addition, and perhaps most significantly, present available dual-scatter back-scatter velocimeters are incapable of measuring the velocity component which lies directly along the axis of the system formed by the bisector of the two beams, thus making a measurement of the vector velocity quite difficult.
The other common configuration for a laser Doppler velocimeter is the "local oscillator" or "on axis" arrangement. Such instruments utilize a single illumination beam focused at the location where particle velocity is to be measured. The scattered radiation is Doppler shifted by particle motion. This frequency-shifted radiation is mixed with radiation of a known frequency on a suitable detector. The frequency difference between the scattered and known radiation provides a measurement of such velocity component.
Most practical local oscillator velocimeters collect the scattered radiation back along the same axis as the transmitted beam. These units measure only the component of the velocity of the particle parallel to the beam axis. Measurement of three velocity components requires a plurality of systems illuminating the sample region from different directions.
U.S. Pat. No. 3,915,572 to Orloff, discloses a combined dual-scatter, local oscillator Doppler velocimeter capable of simultaneously generating signals representative of components of the flow velocity of a liquid in directions both transverse to and along the transmitting axis of the system. This system does not require total duplication of systems in order to measure these two different velocity components. The transverse component in the Orloff patent is measured without reference to the frequency of the transmitted electromagnetic radiation, so that no heterodyning occurs. In the instant invention, as will become clear from the following disclosure, the component of velocity in the transvers direction is determined by a completely different technique. Orloff has simply combined a conventional "dual-scatter" system and a conventional "on-axis" (longitudinal) homodyne system in a novel way.
In many instances, it is desirable to measure the three velocity components of a number of aerosol or other particles in an atmospheric volume from a site remote from that volume. Electromagnetic radiation is usually used as a probe, because it will not adversely effect such measurements. As long as the aerosols are sufficiently small, measurement of the aerosol velocity in effect measures the wind velocity. Because wind velocities are three-dimensional, rather than one- or two-dimensional, it is desirable to obtain a complete measure of all of the velocity components utilizing one instrument. In many instances, the desired wind information cannot be obtained by displacement of on-site anemometers. Such uses as atmospheric boundary layer research, velocities in tornadoes, dynamics of clouds, airport approach path wind shear, aircraft speed, pollution dispersal, ballistics, and wind energy assessment, require remote measurement of velocity components because of the impracticability of utilizing on-site anemometers.
Previous single-beam heterodyne-homodyne Doppler frequency measurements using electromagnetic radiation measure only the line-of-sight component of velocity, not the transverse component. Multiple units have been used independently from sites far separated in angle to measure multiple wind components. The present invention, however, analyzes closely-spaced, multiple signals together on an instantaneous basis, in contrast to the separate frequency analysis used on each beam in the prior art.
In addition, intersecting-beam or dual-scatter "Doppler" velocimeters are well known. However, in the prior art, the return signal is not mixed with a reference signal. The amplitude fluctuations detected yield a frequency proportional to the transverse velocity. If the number of scatterers in the sensing volume is large, the amplitude fluctuations become very small. No information is available on the velocity component parallel to the beam bisector. In contrast, the present invention uses a reference signal to measure the frequency of the return signals and not only their amplitude. Also, heterodyne or homodyne operation is an inherent part of the present invention, whereas dual-scatter systems use only direct detection.